In a radio network system a radio terminal typically transmits a control signal in an uplink transmission to a radio network node. Such a control signal may contain information about the quality of a connection between the radio terminal and the radio network node. In particular, the radio terminal may transmit a channel quality indicator indicating the quality of the radio connection between the radio terminal and the radio network node.
This information on the quality of the connection between the radio terminal and the radio network node is then used at the network node for scheduling further transmission in the uplink and a downlink between the radio terminal and the radio network node.
In this context reciprocity based time division duplex (TDD) systems are of particular interest for future radio network systems such as 5G, since for instance a massive multiple-input multiple-output (MIMO) system, which is a candidate technology for the fifth generation new radio physical layer, may operate in TDD mode.
Massive MIMO, also known as large-scale antenna systems and very large MIMO, is a multi-user MIMO technology where each radio network node may be equipped with a large number of antenna elements, which may be used to serve many radio terminals that share a same time and frequency band and may be separated in a spatial domain. A good assumption is that there are more or even many more BS antennas than terminals, but ideally as many as possible.
Massive MIMO offers many benefits over conventional multi-user MIMO. First, conventional multi-user MIMO is not a scalable technology, since it has been designed to support systems with roughly equal numbers of service antennas and terminals, and relies on frequency-division duplex (FDD) operation.
By contrast, in massive MIMO, the large excess of service antennas over active terminals, TDD operation brings huge improvements in throughput and radiated energy efficiency. These benefits result from spatial multiplexing achieved by appropriately shaping signals sent out and received by the radio network node antennas.
By applying precoding to all antennas the radio network node may cause constructive interference among signals at the locations of the intended radio terminals, and destructive almost everywhere else. Furthermore, as the number of antennas increases, the energy may be focused with high precision into small regions in space.
Other benefits of massive MIMO include use of simple low-power components since it relies on simple signal processing techniques, reduced latency, and robustness against intentional jamming.
By operating in TDD mode, massive MIMO exploits a channel reciprocity property, according to which the channel responses may be the same in both uplink and downlink.
This channel reciprocity may allow the radio network nodes to acquire channel state information (CSI) from pilot sequences transmitted by the terminals in the uplink, and this CSI may be then useful for both the uplink and the downlink, as shown in FIG. 5. In this respect, a pilot sequence may refer to a pattern of reference signals within the used transmission resources.
By virtue of the law of large numbers, the effective scalar channel gain seen by each terminal may be close to a deterministic constant. This may be called channel hardening.
Thanks to channel hardening, the terminals may reliably decode the downlink data using only long-term statistical CSI, making most of the physical layer control signaling redundant, i.e. low-cost CSI acquisition. This may render the conventional resource allocation concepts unnecessary and results in a simplification of the Media Access Control MAC layer. These benefits may have elevated massive MIMO to a good position in preliminary future network system discussions.
However, massive MIMO system performances are affected by some limiting factors. For instance channel reciprocity may require hardware calibration. Further, a so called pilot contamination effect is a basic phenomenon which may profoundly limit the performance of massive MIMO systems. Theoretically, every terminal in a massive MIMO system could be assigned an orthogonal uplink pilot sequence. However, the maximum number of orthogonal pilot sequences that can exist may be upper-bounded by the size of the coherence interval, which may be the product of a coherence time and coherence bandwidth, i.e. the product of the coherence time, i.e. the time duration over which the uplink pilot sequence may be considered unchanged or not varying within a certain range, and the coherence bandwidth, i.e. a statistical measurement of a range of frequencies over which the pilot sequence may be considered flat, or in other words an approximate maximum bandwidth or frequency interval over which two frequencies of the pilot sequence are likely to experience comparable or correlated amplitude fading.
Hence, adopting orthogonal pilot sequences may lead to inefficient resource allocation as the number of the terminals increases or it may not physically be possible to perform when the coherence interval is too short.
As a consequence, pilot sequences may be reused across cells of the radio network system or even within a home cell for instance for higher cell density. This may cause interference among terminals which share the same pilot.
Pilot contamination does not vanish as the number of radio network node antennas grows large, and so it is one impairment that remains asymptotically.
Further, reciprocity based beamforming may rely on accurate channel state information at a transmitter (CSI-T) such as the radio terminal. In case the number of transmit antennas is significantly larger than the number of receive antennas, as is the case in TDD massive MIMO downlink scenario, then the CSI-T may be efficiently obtained by transmission of SRS in the reverse link, i.e. in the direction from the radio terminal to the network node.
The required periodicity of SRS may depend on a coherence time of the radio channel, which in turn depends on the speed of movement of the radio terminal.
Therefore, there is the need to ensure the required periodicity of SRS in a radio network system.
Thus, an object of the present invention is to ensure accurate transmission quality information is used when transmission timing is assigned to a radio terminal. Another object of the present invention is to enable reciprocity based acquisition of transmission quality information for assigning transmission timing to a radio terminal.
Yet another object of the present invention is to reduce the reference signal overhead for assigning transmission timing to a radio terminal.